Conventionally, the CRT of a well known type has an electrode structure consisting of a cathode K, a primary grid G1, a secondary grid G2 and an anode A. By using the CRT of this type, an applicant of this invention has developed such a circuit means that control signals like a video signal is entered to the cathode K while a focus voltage developed on a secondary side winding of a flyback transformer is applied to the anode A, and then has filed Japanese Patent Applications Nos. 62-93208 and 62-93209. In the circuit means, a horizontal drive pulse is supplied to the primary side of the flyback transformer so that a flyback pulse resonated by a resonance capacitor and a deflecting yoke during the horizontal retrace period is supplied to a primary side winding of the flyback transformer while the deflecting yoke is driven. To this end, a necessary focus voltage is generated on the secondary side winding of the flyback transformer.
In the case the flyback transformer has an unstable supply voltage on the primary side thereof, an error caused due to a supply voltage variation on the primary side gives influences through the flyback transformer on an output generated on the secondary side.
High voltage which is applied to the anode of the CRT (hereinafter referred to as "focus voltage") is under the influence of the error. Even though the primary side supply voltage has only the error of .+-.0.2V, an anode end connected to the secondary side will have the error of about .+-.80V. To this end, a fluctuation of a focal point is caused to change a beam spot diameter, resulting in ill effect.
In order to prevent the influence of the error mentioned above, there is conventionally provided such a means that a voltage sufficiently stabilized by using a three-terminal regulator is used as the primary side supply voltage.
However, with the conventional means for providing the stabilized supply voltage to be used as the supply voltage for the primary side winding, power consumption is increased as a result of current consumption in the three-terminal regulator per se and of its voltage drop between input and output potentials.
Further, stabilization of only of the primary side supply voltage by means of the three-terminal regulator cannot perform a fine adjustment of the focal point.